Method of producing a fibrous web material having retained wet strength at high humidity and the fibrous material produced thereby



United States Patent 3,468,696 METHOD OF PRODUCING A FIBROUS WEB MA- TERIAL HAVING RETAINED WET STRENGTH AT HIGH HUMIDITY AND THE FIBROUS MA- TERIAL PRODUCED THEREBY Bernard W. Conway, Holyoke, Mass. C. H. Dexter &

Sons, Inc., Windsor Locks, Conn. 06096) V N0 Drawing. Filed Apr. 5, 1965, Ser. No. 445,796 Int. Cl. B44d N44 US. Cl. 11762.1 Claims ABSTRACT OF THE DISCLOSURE A method of providing a fibrous web material with enhanced wet strength and retained wet strength at high humidity comprises the steps of treating a preformed fibrous web material with an aqueous alkaline solution containing carboxymethylcellulose and a water-soluble reaction product of epichlorohydrin and a polyamide, drying the alkaline treated web, subsequently acidifying the dried web with a dilute acid solution and then waterwashing the fibrous Web material to remove the nonsubstantive acid therefrom. The pH of the alkaline treating solution is between about 9 and 11 while the waterwashed web exhibits a paper pH within the range of 4 through 6 as determined by TAPPI test method T435-m- 52.

The present invention relates to a fibrous web material exhibiting improved wet strength characteristics and to the method of its manufacture.

In recent years increased emphasis has been placed upon the use of wet strength resins which are operable in the alkaline or neutral pH range. Such resins, commonly referred to as alkaline curing resins, are particularly desirable since they permit machine operation under the less corrosive, nonacid conditions while at the same time imparting improved strength and softness to the paper and permitting the use of alkaline fillers and the {like The introduction of these alkaline curing wet strength resins to the industry has met with varying degrees of success. Predominant among those presently in use are the water-soluble cationic resins falling under the general category of epoxidized polyamides. The epoxidized materials broadly consist of polymeric reaction products of epichlorohyrin and a polyamide. These resins are generally thermo-setting materials which may be utilized under alkaline conditions, applied to the fibers in dilute aqueous suspensions and finally cured to a waterinsoluble state. The resins have also been employed in conjunction with relatively small amounts of certain water-soluble gums to advantageously improve the wet strength of paper products. However, when employed in conjunction with such gums and when used as a postformation treatment of the paper, it has been found that the papers tended to lose a substantial degree of their Wet strength at high humidity conditions.

Accordingly it is an object of the present invention to improve the wet strength of papers treated with alkaline curing resins and particularly to impart thereto the ability to retain their wet strength at high humidity for prolonged periods of time.

Another object of the present invention is to provide a new and improved process for imparting to a paper treated with an alkaline curing resin an unexpected increase in the wet strength thereof in a relatively simple, rapid, facile, and economical manner.

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A further object of the present invention is to provide a paper exhibiting not only improved initial and prolonged wet strength and retained wet strength at high humidity but also a paper which exhibits the characteristics of acid curing treatments while at the same time retaining the desirable characteristics of alkaline curing wet strength resins.

Other objects will be in part obvious and in part pointed out more in detail hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with re spect to each of the others and the article possessing the features, properties, and the relation of elements, which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

In carrying out the process of the present invention a fibrous paper is initially produced in the form of a continuous web material in accordance with known and conventional papermaking techniques. The paper web material is subjected to a post-formation wet strengthening treatment with a solution of an alkaline curing cationic resin and a water-soluble polymeric material possessing active hydrogen groups, such as a water-soluble gum, following which the resin coated paper is dried and the pH of the resultant web is lowered to 6.0 or less prior to final washing, pressing, and drying operations.

The cationic resin and the polymeric material are generally admixed prior to their application to the preformed fibrous web. The admixture desirably takes the form of a dilute aqueous solution having an alkaline pH and may be prepared by dissolving the resin and Water-soluble polymeric material in an alkaline solution, such as a dilute sodium hydroxide solution, and then adjusting both the concentration of the components and pH of the solution to attain the desired results. The pH of the solution is generally maintained in the medium to high alkaline range, namely above about 9.0, with the preferred operating pH being approximately 10 to 11. As shown in Example Six herein, papers treated with a resin/gum solution having a pH on the acid side exhibit a degradation in strength when further treated in accordance with the present invention.

The concentration of the components in the alkaline solution will vary somewhat depending on the particular web being treated as well as on the particular alkaline curing resin and water-soluble gum employed. In general it has been found that satisfactory results can be achieved at various gum to resin ratios, however, in practice this ratio is usually maintained at greater than 1:1, ratios of approximately 2:1 to 15:1 giving excellent results. The concentration of the water-soluble gum in the alkaline solution may vary from about 0.1 percent by weight up to about 10.0 percent by weight. However, the preferred concentration is from about 0.5 to 5.0 percent by weight and, more specifically, 1.0 to 2.0 percent. Amounts in excess of 10.0 percent are generally not employed since the increased amount does not result in a proportional increase in the strength of the web treated thereby. The alkaline curing resin may be effectively used in concentrations up to about 5.0 percent by weight or more. Generally an operating range from about 0.01 to about 1.0 percent has been found suitable with the preferred range being about 0.05 to 0.5 percent by weight.

The cationic alkaline curing resins suitable for use in accordance with the present invention are preferably the uncured thermosetting epoxidized polyamide resins, exemplary of which are the resins described in United States Patent Nos. 2,926,116 and 3,125,552. The resins are watersoluble polymeric reaction products of epichlorohydrin and polyamides containing secondary amine groups. The epichlorohydrin acts in a manner similar to a cross-linking agent by reacting with secondary amine groups within the polyamides resulting in a conversion of the secondary amines to tertiary amines, the vicinal epoxy ring being available for the cross-linking process. Generally, polyamides derived from polyalkylene polyamines and saturated or unsaturated aliphatic or aromatic polycarboxylic acids containing from 3 to carbon atom are preferred. A typical example of such material is the watersoluble epichlorohydrin-polyamide reaction product sold by Hercules Powder Company, Wilmington Delaware, under the name Kymene 557, the particular polyamide used being derived from diethylene triarnine and adipicacid.

As mentioned hereinbefore the water-soluble polymeric material possesses active hydrogen groups or, more specifically, groups such as carboxly, hydroxyl or amino. Typical of such materials are the water-soluble gums which include the water-soluble cellulose ethers such as methyl cellulose or the various types of sodium carboxymethylcellulose (CMC), sometimes referred to as sodium cellulose glycolate.

The paper or fibrous sheet material is generally formed in the usual papermaking fashion in the form of long, continuous webs which can be easily rolled up on collecting spools. These preformed fibrous webs, after partial or complete drying, are treated with the alkaline solution in accordance with conventional solution treatment techniques such as by tub application. In this regard excellent results have been obtained utilizing an immersion or dipcoating process to obtain the desired impregnation of the web. The coated or impregnated web is then dried and, in accordance with the present invention, subjected to an acid treating step which imparts to the sheet material the desired improvement in wet strength while lowering the paper pH to approximately 6.0 or less.

The acid bath or treatment is generally applied by tub application although most coating techniques may be employed with good results. The bath generally consists of a dilute solution of an inorganic or organic acid or acid salt having a concentration of about 5.0 percent by weight or less. The acids utilized may be the common mineral or inorganic acids such as hydrochloric or sulfuric acid; the organic acids such as acetic, fumaric or lactic acid; or the acid producing salts such as aluminum sulfate, ammonium chloride or the like. The preferred acid solutions are generally of a concentration greater than approximately 0.01 and less than about 1.0 percent by weight, the exact utilizable range varying with the type of acid employed. It will of course be appreciated that acid concentrations higher than those set forth herein may be utilized where the web is subsequently washed for an extended period of time. For example, the strong mineral acids, such as sulfuric acid, may be utilized at concentrations greater than 0.5 percent and yield excellent results where the paper pH is about 5.0, see Example Two, while at concentrations of less than 0.20 percent by weight yield poor results where the paper pH is about 3.5, as evidenced by Example Seven.

The acid treatment markedly increases the acidity of the ultimate product and preferably results in a paper 'web material whose extract possesses a pH of 6.0 or less. However, when the pH of mineral acid treated papers drops below about 3.5 the acidity of the web causes degradation in the Strength and structural integrity of the web, it being understood, of course, that organic acid treated papers may exhibit a lower pH and still retain their high strength characteristics due to their weaker activity.

After the acid treatment the web is generally washed with water for a brief interval prior to drying and collection in the conventional manner. The principal purpose of the washing operation is to remove the excess nonsubstantive materials and particularly the excess acid from the web before it passes through the heated driers. The washing time is generally kept to a minimum, however, so as not to completely leach out the acid applied to the web.

The term ream weight as used herein refers to the weight in pounds of 480 sheets each being 24 inches wide and 36 inches long, in accordance with TAPPI test method T410-os-61 or its related standard.

The term paper pH as used herein refers to the pH as determined by TAPPI test method T435-m-52 for Hydrogen Ion Concentration of Paper Extracts in accordance with the cold extraction procedure. That procedure consists of placing 1.0 gram of air dry paper in a ml. beaker to which is added 20 ml. of distilled water. The paper is macerated with a glass rod until uniformly wet after which an additional 50 ml. of water is added to the beaker. The beaker is then covered and allowed to stand for one hour. At the end of this time the solution is stirred and the pH of the unfiltered mixture is measured at room temperature.

The wet tensile strength values set forth herein have been obtained according to TAPPI test method T456- m-49. This method requires that the paper being tested be completely saturated with water at room temperature prior to measuring the tensile strength according to TAPPI test method T404-os-61.

Having generally described the invention, the following examples are included for purposes of illustration so that the invention may be more readily understood and are in no way intended to limit the scope of the invention unless otherwise specifically indicated. All amounts are on a weight basis unless otherwise specified.

Example One A preformed fibrous sheet having a ream weight of 14.5 pounds and comprising 100 percent hemp fibers was dip-coated with an aqueous alkaline solution having a pH of 10.0, a sodium carboxymethylcellulose (CMC) concentration of 1.0 percent by weight and about 0.1 percent by weight of a polymeric reaction product of epichlorohydrin and a polyamide (Kymene 557). The sheeet was then dried. A first portion of the treated sheet, designated l-A, was simply washed with water and dried. A second portion of the treated sheet, designated l-B, was dipped in a 1.0 percent hydrochloric acid solution, Washed with water, pressed, and dried.

Both sheets were subsequently tested for wet tensile strength and paper pH in accordance with standard TAPPI test methods. Sheet l-A had a paper pH of 6.2 and a wet tensile strength of 922 grams per inch width while the acid treated portion, sheet l-B, had a paper extract pH value of 5.6 and a wet strength of 1040. After permitting both sheets to age for 23 months at room temperature, the wet strength of sheet l-A had dropped to 842 grams per inch width while the wet strength of the acid treated paper had increased to 1500 grams per inch width. Thus the tests indicated not only a higher initial wet strength for the acid treated paper but also a substantial increase in the wet strength of the paper upon aging.

Example Two A 7% pound ream weight paper was produced from a furnish containing about /3 wood and /3 hemp fibers and a minor amount, less than 1.0 percent by weight, of an epoxidized polyamide (Kymene 557) and karaya gum. Following the procedure of Example One, test samples of the paper were all dip-coated in dilute aqueous solutions, treated with the acid solutions as indicated in Table I and tested. Particularly significant are the wet tensile strengths before and after storage at high humidity.

TABLE I Examples Resin/CMC Treatment:

Resin Conc., percent. CMC Conc., percent Solution, pH

Acidr Treatment:

YP Conc., PAPE R pH v Wet Tensile Strength (gmJrn.

width):

At time oi treatment After ambient cure for about 3 Weeks After 3 weeks ambient cure plus 1 week at 95% humidity and 120 F After ambient cure for 17 months Aiter ambient cure for 21 months Example Three A 7 pound ream weight paper comprising /3 wood and 73 hemp fibers and no beater additive was treated with aqueous alkaline solutions containing CMC and an epoxidized polyamide resin (Kymene 557) prior to dipresin solutions are given in Table III along with the test results.

TABLE III Examples 4-A 4-13 4-0 4-D Resin/CMC Treatment:

Resin Conc., percent 0.3 0.3 0.1 0. 1

CMC Conc., percent 1. 5 1. 5 1. 5 1. 5 Acid Treatment No Yes No Yes Paper, pH 7. 5. 7. 45 5. Wet Tensile Strength (gmJin. width):

At time of treatment 732 755 467 572 Oven cure at 350 F. for 10 min--. 832 1, 745 881 Example Five TABLE IV Wet Tensile Strength Resin/CMC Treatment Acid Treatment (gm. in. width) Dry Ten- Resin CMC At time Oven cure Strength Conc., Conc Solution Conc., Paper, of treatat 359 F. (gm./in percent percent pH Type percent pH ment for 10 min. width) 0. 5 1. 0 10. 2 H2804 0.096 5. a 1,010 1, 235 3, 540 0. 5 1. 0 10. 2 E2504 0. 192 3. 6 910 495 2, 690

coating in a dilute acid solution. The treatments and results are tabulated in Table H.

TABLE II Resin/CMC Treatment Wet Tensile Strength (gmJin. width) Acid Treatment Resin CMC At time Ambient Conc., Conc., Solution Conc., Paper, of treat cure for Oven Humidity Ex percent percent pH Type percent pH ment 2 months cure I test 1 3- 0.09 1.2 8.4 0. 09 1. 2 5. 3 0. 12 1. 2 7. 5 0. l2 1. 2 6. 0 0. 12 1. 2 5. 0 0. 17 1. 5 7. 0 0. l7 1. 6 10. 5 H2304 0. 032 5. 8 0. 17 1. 5 10. 5 H2504 0. 16 5. 3

1 Cured for 10 min. at 350 F. 2 Samples kept at 80% relative humidity and 76 F. for one week.

Example Four This example illustrates the improved result obtained from the acid treatment of the present invention when the concentration ratio between the resin and the watersoluble gum is substantially varied.

Separate portions of a 100 percent hemp fiber web having a ream weight of 12.85 pounds was treated with aqueous alkaline solutions having gum to resin ratios of 5:1 and 15:1. All other factors were kept substantially identical. The alkaline solutions each had a pH of 10.0 and each acid treated sheet was dip-coated in a 0.064% sulfuric acid solution. The concentrations of the alkaline Example Six This example illustrates the effect of utilizing an acid solution of gum/ resin in place of the highly alkaline solution of the present invention.

Paper samples from a hemp fiber web having a ream weight of 12 pounds were treated with a solution of 1.0 percent by weight CMC and 0.1 percent by weight epoxidized polyamide (Kymene 557) having the indicated solution pH. The test results set forth in Table V indicate the necessity of using an alkaline rather than an acid solution.

TABLE V Wet Strength Tensile Dry Ten- Example Seven This example illustrates the beneficial result obtained from using organic as well as inorganic acids.

A hemp paper web having a ream weight of 12 pounds was treated with an alkaline solution having a pH of 10.2 and comprising 1.0 percent by weight CMC and 0.1 percent by weight epoxidized polyamide (Kymene 557"). The treated web was dried and samples thereof were acid treated as indicated in Table VI.

As will be apparent to persons skilled in the art, various modifications and adaptations of the process and product above-described will become readily apparent without departure from the spirit and scope of the invention.

I claim:

1. A method of improving the wet strength characteristics of fibrous Web materials while preventing degradation of the dry strength thereof comprising the steps of treating a preformed fibrous web material with an aqueous alkaline solution comprising a water-soluble gum and a water-soluble alkaline curing resin, said solution being maintained in the medium to high alkaline range; and acidifying the alkaline treated web with a dilute acid solution to produce a fibrous web material having enhanced wet strength, retained wet strength at high humidity and a paper pH within the range of 3 to 6 as determined by TAPPI test method T435am-52.

2. A method of improving the wet strength characteristics of fibrous web materials while preventing degradation of the dry strength thereof comprising the steps of treating a preformed fibrous web material with an aqueous alkaline solution comprising a water-soluble cellulose ether and a Water-soluble thermosetting alkaline curing resin, said solution having a pH of above about 9; curing the resin on the web; acidifying the alkaline treated web with a dilute acid solution having an acid concentration of less than about 5 percent by weight; and subsequently water-washing the acidified web to produce a fibrous web material having enhanced wet strength, retained wet strength at high humidity and a paper pH within the range of 3 to 6 as determined by TAPPI test method T435-m-52.

3. A method of improving the Wet strength characteristics of fibrous Web materials while preventing degradation of the dry strength thereof comprising the steps of treating a preformed fibrous web material with an aqueous alkaline solution comprising about 0.5 to 5.0 percent by weight of a Water-soluble cellulose ether and about 0.01 to 1.0 percent by weight of a water-soluble reaction product of epichlorohydrin and a polyamide, said solution having a pH within the range of 9 to 11; drying the treated web; acidifying the dried alkaline treated web with a dilute acid solution having an acid concentration of about 0.01 to 5.0 percent by weight; and subsequently Water-washing the acidified web to produce a fibrous web material having enhanced wet strength, retained wet strength at high humidity and a paper pH within the range of 4 to 6 as determined by TAPPI test method T435-m-52.

4. A method of improving the wet strength characteristics of fibrous web materials while preventing degradation of the dry strength thereof comprising the steps of treating a preformed fibrous web material with an aqueous alkaline solution comprising carboxymethylcellulose and a water-soluble thermosetting reaction product of epichlorohydrin and a polyamide, said solution having a pH within the range of 9 to 11; acidifying the alkaline treated Web with a dilute acid solution; and subsequently water-washing the acidified web to produce a fibrous web material having enhanced wet strength, retained wet strength at high humidity and a paper pH within the range of 4 to 6 as determined by TAPPI test method T435-m-52.

5. A method of improving the wet strength characteristics of fibrous web materials while preventing degra dation of the dry strength thereof comprising the steps of treating a preformed fibrous web material with an aqueous alkaline solution comprising about 0.5 to 5.0 percent by weight of a water-soluble cellulose ether and about 0.01 to 1.0 percent by weight of a water-soluble resin reaction product of epichlorohydrin and a polyamide containing secondary amine groups, the ratio of ether to resin being greater than 1:1, said solution having a pH Within the range of 10 to 11; drying the treated Web; and acidifying the dried alkaline treated web with a dilute acid solution having an acid concentration of about 0.01 to 1.0 percent by weight to produce a fibrous web material having enhanced wet strength, retained wet strength at high humidity and a paper pH within the range of 4 to 6 as determined by TAPPI test method T435-m-52.

6. A method of improving the wet strength characteristics of fibrous web materials while preventing degradation of the dry strength thereof comprising the steps of treating a preformed fibrous web material with an aqueous alkaline solution comprising about 1.0 to 2.0 percent by weight of carboxymethylcellulose and about 0.05 to 0.5 percent by Weight of a Water-soluble reaction product of epichlorohydrin and a polyamide containing secondary amine groups, said reaction product having only tertiary amine groups, said solution having a pH within the range of 10 to 11; drying the treated web; acidifying the dried alkaline treated web with a dilute acid solution having an acid concentration of about 0.01 to 1.0 percent by weight; and subsequently water-Washing the acidified web to produce a fibrous web material having enhanced Wet strength, retained wet strength at high humidity and a paper pH within the range of 4 to 6 as determined by TAPPI test method T435-m-52.

7. A fibrous web material treated in accordance with the method of claim 1.

8. A fibrous Web material treated in accordance with the method of claim 3.

9. A fibrous web material treated in accordance with the method of claim 4.

10. A fibrous web material treated in accordance with the method of claim 6.

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3,248,280 4/1966 Hyland 117-155 X 3,259,600 7/1966 Coscia et al.

3,332,901 7/1967 Kein 117-155 X 3,058,873 10/1962 Kein et al 162-164 3,066,066 11/1962 Kein et al. 162-164 X WILLIAM D. MARTIN, Primary Examiner M. LUSIGNAN, Assistant Examiner US. Cl. X.R. 260-97, 97.5, 

